1. Field of the Invention
The present invention relates to a zoom lens system, and more particularly, to a zoom lens system preferable for use as an image-taking optical system of a camera.
2. Description of the Related Art
In recent years, more compact image-taking optical systems with higher performance are needed as image-taking systems for cameras.
Especially for an image-taking optical system of a lens shutter camera, a reduced size of such a camera is becoming possible with the aid of technical progress of associated mechanisms and electrical circuits, so that a zoom lens system achieving a higher zoom ratio and a smaller size at the same time is desired as an optical system of the camera.
As an image-taking optical system for a lens shutter camera, Japanese Patent Application Laid-Open No. H8(1996)-262325 (corresponding to U.S. Pat. No. 5,721,643), Japanese Patent Application Laid-Open No. H9(1997)-120028 (corresponding to U.S. Pat. No. 5,793,535) and the like have proposed a zoom lens which is formed of three lens units having a positive refractive power, a positive refractive power, and a negative refractive power, respectively, in order from an object side.
Japanese Patent Application Laid-Open No. H9(1997)-15499 (corresponding to U.S. Pat. No. 5,587,840) and Japanese Patent Application Laid-Open No. H10(1998)-031155 have also proposed a zoom lens which provides a zoom ratio of approximately three to four and is formed of four lens units having a negative, a positive, a positive, and a negative refractive powers, in order from an object side.
In addition, Japanese Patent Application Laid-Open No. H4(1992)-338910, Japanese Patent Application Laid-Open No. H8(1996)-220438 and the like have proposed a focusing method for reducing variations in aberration when images of an object at a short distance are taken.
Japanese Patent Application Laid-Open No. H3(1991)-249614 (corresponding to U.S. Pat. No. 5,172,273) has proposed a zoom lens system which is formed of four lens units having a positive, a positive, a positive, and a negative refractive powers in order from an object side, as well as a method of moving a second lens unit and a third lens unit together and a method of moving a fourth lens unit as a focusing method using one or more lens units other than a first lens unit.
When attempts are made to realize a zoom lens system which provides a higher zoom ratio and has a smaller size, however, optical performance is likely to change at the time of focusing. It is difficult to achieve favorable optical performance over the entire zoom range while such changes in optical performance are suppressed.